134 RIGHT International, 15 January 1977
TOMORROW'S POWER
•4- page 116
An improvement of between 3y and 5j per cent
in specific air range of the TriStar is reported
as a resuit of fitting the RB.2I I with a 15
afterbody in place of.the original 11c design
approach might also make it possible to dispense with,
or greatly reduce, the nacelle acoustic lining. The length
of the fan nacelle could therefore be much less, its
design being governed by various performance requirer
ments, and the need to engineer a thrust-reverser might
be eliminated by using reversible-pitch fan blades or even
by relying on the fan's substantial idling drag.
With the considerable reduction of installation drag
and weight offered by a "minimum cowl," and the de
coupling of the fan and 1-p turbine design constraints, we
could then go for a further significant reduction in a
specific thrust to about 50N/kg/sec—about one-third of
the current figure. The corresponding bypass ratio is
about 20, with fan pressure ratio reduced to 1-25 from
the present value of about 1 • 6. Cycle pressure ratio would
be similar to the previous example, with perhaps a
50-100°C further increase in turbine-entry temperature.
Although the engine itself would be about 70 per cent
larger in diameter than a current turbofan for a given
cruise thrust, the overall diameter would not increase
as much as this because of the "minimum cowl" design
approach. With similar provisos to those given for the
first example, the installed s.f.c. of this class of power-
plant should be perhaps 20 per cent better than the cur
rent turbofan figure. Both of these turbofan concepts
would make extensive use of composites in the nacelle.
No return to the propeller
The third example represents a more radical change.
The cowl is discarded altogether and we return to the
open propeller. But this is a "propeller with a difference."
It embodies transonic aerodynamic technology and
multiple blades to achieve both a high disc loading and
a reasonable efficiency at flight Mach numbers up to
about 0-8. Six or eight blades might be used, swept back
towards the tips and featuring an advanced form of
construction with a spar inside each blade. Estimates
for the fuel consumption of such powerplants make them
as much as 20 per cent more economical than present
engines, depending on the efficiency of the "prop-fan"
itself and the cycle used for the gas generator. Becent
Hamilton Standard tests of a model prop-fan in a wind
tunnel indicate an efficiency of 0-77 at Mach 0-8 and
suggest that the target efficiency figure of 0-8 used in
project studies should be attainable. Although the prop-
fan has some important potential drawbacks—lower
standard of ride comfort; possibly higher cabin noise at
Figure 6: Roll-Hoyce's geared mid-fan scheme
cruise; higher weight; cruising speed limit of around
Mach 0-8—its attractive fuel economy constitutes an
incentive for further research and evaluation.
It is too early to forecast with confidence which of
these powerplants is most likely to materialise, or when
a project might be launched. My personal view is that
we are rather unlikely to return to propulsion by open
rotor for the larger transport aircaft, and that some type
of advanced tubofan will form the powerplant of the
early 21st century. The fan cowl and outlet stator blades
perform very valuable functions. Together they control the
flow of the propulsive airstream, largely removing exit
swirl and its associated losses, and minimising the
periodic flow interactions with the airframe which give
rise to vibration. Some of the design problems of future
turbofans may look formidable now, but it is worth
remembering that serious attempts to predict future
development have quite often turned out to be pessi
mistic. A good example is a belief which prevailed quite
widely at the beginning of the 1960s. It was then felt
that bypass ratios for main propulsion engines were
unlikely to rise above a value of about two, at least
for a very long time, due to the weight and installation-
drag penalties which seemed inevitable with lower-
specific-thrust designs. In the event, advances in high-
temperature technology, aerodynamics, materials and
mechanical design led within that decade to the launch
ing of the current large engines, which have bypass ratios
of about five.
Much time has been devoted to studying alternative
layouts aimed at economising on the total number of
turbo-machinery blade rows required. Figure 6 shows the
geared mid-fan engine recently studied by Bolls-Boyce.
Its low-pressure turbine is divided into two sections: a
high-speed forward portion is geared to a lower-speed
rear section mounted on a rotating casing which also
carries the fan blades. This scheme bears some relation
to the "Bostat" idea put forward by Howell at NGTE
in the 1940s. The latter featured rotation of the turbine
"stator" blading (hence "Bostat") but did not embody
the gear system. Although such schemes offer a saving
in the number of blade rows required, and also remove
certain gas-generator design constraints because of the
absence of an 1-p shaft passing through the system, they
introduce problems associated with the sealing of the
rotating casing.
Conclusion In the 50 years which have elapsed since
Griffith's "first practical proposal to use a gas turbine
'.:.'," the aero-engine world has undergone a vast trans
formation. The gas turbine quickly fulfilled its promise of
high unit power and light weight. The BAE-inspired
development of the axial compressor provided a basis
for today's efficient, high-pressure-ratio engines. Gas-
turbine powerplants now appear in a variety of forms
to suit the wide range of modern propulsion require
ments.
For the future, we can certainly anticipate a con
tinuation of intensive and highly competitive develop
ment under the spur of military requirements and the
huge business potential of civil air transport. Technically,
there is much still to come. As ever, we face an array
of opportunities and problems. There is a continuing
need for design ingenuity and for perseverance in
research and development. With much at stake, good
judgement of both engineering and commercial issues
will be vital.